WOODSTOCK – Three stray dogs sheltered at the McHenry County Animal Control and Adoption Center have tested positive for heartworm, according to the McHenry County Department of Health.

The positive tests in recent weeks are more than the prior six months combined, according to a news release. Heartworm for animals is the equivalent to the West Nile virus in humans, which is most commonly caused by mosquitoes.

The dogs were brought to the shelter as strays from Algonquin, Woodstock and Crystal Lake. It is believed mosquitoes infected them at least six months ago.

Heartworm cannot pass from dog to dog but is transmitted by mosquitoes, which the area has seen a dramatic increase in due to recent wet, humid weather. An annual blood test for heartworm is recommended for dogs.

It can take up to six months from the time an infection occurs before the test for heartworm is positive due to the fact it lives inside the circulatory system, not the intestine.

Veterinarians encourage medication be used once a month on dogs to prevent the disease.

Failure to treat infected dogs can lead to death from congestive heart failure.